I have compared two well-exposed flatfields in the B-filter from the dusk session night JD2455856 with a time difference of about 28.5 minutes.

Each flatfield was bias-subtracted with the proper scaled superbias, had a fitted surface subtracted, and was then normalized. A percent difference image was created as the difference compared to the earlier flatfield.
perc = ((late-early)/early)*100

The mean of this image is of course very close to zero (0.0014), but more interesting is the standard deviation (0.53).

The strongest part of the familiar diagonal pattern is still visible in the percent image (see left part of figure). I have used Image J to rotate the percent image and plot the horizontal profile of the yellow box (top right part of the figure) selected to be perpendicular to the diagonal darker areas. This profile is plotted in the bottom right part of the figure.
It can be seen that the diagonal structure is 0.2pp darker than the immediate neighborhood.

The diagonal structure does change within a relatively short time-frame and this will result in an uncertainty in the science frames when they are flatfield corrected. So far it doesn’t seem to be a large change, but more investigations are necessary. The more likely explanation for the change is temperature fluctuations. Perhaps it is possible to investigate if the diagonal structure changes with a period comparable to the change in bias level…